Try the school, its alumni association, the local library, county historical societies, state archives, digitised collections and former classmates who own copies.
It confirms attendance and appearance, shows activities and identifies classmates, teachers and friends who may never have been interviewed.
They show what someone wrote at the time, which can reveal relationships, but they should be corroborated rather than treated as proven facts.
Avoid it without strong reason, since they are unrelated people, and remember yearbook photographs are usually owned by the school or studio.
Note the school, academic year, page number and owner of the copy, and log each item in a source ledger with scans.